1. Field of the Invention
The present invention relates to a method of manufacturing an ion current recording head used as a recording head of an electrostatic plotter or the like.
2. Description of the Related Art
An ion current recording head basically comprises an ion source and an ion current control member for controlling the flow of ions. The ion current control member is obtained by forming a large number of small holes for allowing ion currents to pass therethrough in a printed circuit board constituted by a solid insulating body having electrodes arranged on its upper and lower surfaces. Ions generated by the ion source pass through the ion through holes of the ion control member and are radiated on a recording medium. At this time, a predetermined voltage pulse is applied to the electrodes on the upper and lower surfaces of the ion current control member so as to change the direction and intensity of an electric field in each small hole, thus controlling the amount of ions passing therethrough. As a result, a desired electrostatic latent image is recorded in the recording medium, and is developed later to form a desired image.
In such a recording head, in order to form a uniform image, a large number of ion through holes are required to have uniform size and shape. This causes great difficulty in the manufacture of an ion current recording head. The larger the width of an image to be formed, to which a recording head is applied, the greater the difficulty in manufacture. For example, a recording head to be applied to formation of an A.phi. image having a width of 840 mm at a recording density of 8 dot/mm is required to have at least 6,720 uniform ion through holes. In addition, a head to be applied to gray scale recording requires higher precision. For example, in a head to be applied to 64-level recording, a variation in area of ion through holes must be 1.5% or less.
As a method of forming ion through holes in a printed board, Published Unexamined Japanese Patent Application No. 61-224487 discloses a method of forming holes in a board using a microdrill having a diameter of 100 to 300 .mu.m. The method of forming ion through holes by means of drilling takes a very long period of time to form a large number of ion through holes. In addition, burrs are produced at the opening of the metal electrodes, and the positional precision of the holes is low. Furthermore, if a drill is broken on the course of formation of a large number of ion through holes, an incompleted recording head cannot be used any more, and the previous process is wasted. Moreover, since a small-diameter microdrill to be used has poor durability, early replacement of the microdrill is required, resulting in an increase in cost.
As another method, Published Unexamined Japanese Patent Application Nos. 61-224489 and 61-224490 disclose a method of forming ion through holes in a printed circuit board by etching. In a normal printed board, a copper film is bonded to an insulating board with an adhesive agent. As this adhesive agent, a material having high etching resistance is mainly used. Consequently, an adhesive agent having high etching resistance is interposed between a board and a copper film. As a result, ion through holes cannot be formed in the printed board depending on a type of etching. According to another available board, a thin metal film is formed on an insulating board by sputtering without using an adhesive agent. In a board of this type, however, since adhesion between a metal film and the board is small, the metal film may be peeled off when soldering is performed to connect an electric circuit system to a recording head, a connector is inserted, or etching is performed.
An adhesion-reinforced sputtering board is available as another board using no adhesive agent. This board has large adhesion between a thin metal film and an insulating board. As such a board, a polyimide board with a copper film is available from Mitsui Mining & Smelting Co. Ltd., which is obtained by forming a copper film on a polyimide board. When this board is immersed in an etching solution, the polyimide board is isotropically etched from portions exposed by a small hole pattern formed in the copper film. More specifically, the exposed portions are etched, from the upper and lower surfaces of the polyimide board, in almost semispherical shapes having the holes formed opposite in the upper and lower surfaces as the centers. Therefore, the inner surface of each ion through hole formed in this manner is not flat but has a ring-like projection midway along each hole. In addition, each portion under the metal pattern is etched to have a larger diameter than a corresponding hole of the pattern. For this reason, if etching is insufficiently performed, a portion of the polyimide board is left midway along each through hole in the form of a ring. When such a board is used as a recording head, this ring-like portion is charged with ions to cause reduction in ion current. If etching is excessively performed, copper films around the through holes collapse. As a result, electric fields may be changed when a voltage plus is applied to the electrodes, or the electrodes on the upper and lower surfaces may be short-circuited. In addition, the use of a special sputtering board such as a polyimide board with a copper film causes an increase in cost. Moreover, the shape of each ion through hole to be formed by etching is greatly influenced by the direction, flow rate, and temperature of an etching solution, and it is impossible to set these conditions to be the same for each hole. Hence, holes having different shapes are inevitably formed.
Micropatterning using laser heat is employed as a means for forming through holes in an insulating body (polyimide board). Hole formation in an insulating body was actually performed by using a YAG laser and a CO.sub.2 laser. In hole formation using an Al.sub.2 O.sub.3 ceramic board, Al.sub.2 O.sub.3 melted by heat was scattered around each hole to produce burrs. In hole formation using a polyimide board, holes were formed into indefinite shapes due to heat fusion, and hence the resultant structure was not suitable for practical use.